Automatic control of amplification



I Oct. 24, 1939. H, BARTELS 2,177,050

AUTOMATIC CONTROL OF AMPLIFICATION Filed Oct. 29, 1955 FfOl/E/VC Y (nun VIIIII" INVENTOR HANS BARTELS ATTORNEY v Patented Oct. 24, 1939 PATENT OFFICE AUTOMATIC CONTROL OF AMPLIFICATION Hans Bartels, Berlin-Spandau, Germany, assignor to Telefunken Gesellschaft fiirDrahtlose Telegraphic in. b. H., Berlin,

tion of Germany Germany, a corpora- Applica tion October-29, 1935, Serial No. 47,265 In Germany November 10, 1934 1 Claim.

. This invention relates to means for automatically controlling the amplification ratio of am.- plifiers utilized in connection with telephony, sound recording and reproducing, radio broadcasting and the like. .It has for its principal object the provision of an improved apparatus and method of operation whereby the amplifies.-

tion ratio is controlled by the signal amplitude or volume modified in accordance with the respouse of the human ear to tones or sounds of different frequencies.

Experiments have shown frequency dependent control modified in accordance with the frequency sensitivity curve of the ear. to be of advantage v in case of automatic control of the amplification ratio. An auxiliary amplifier may be provided to produce a direct current control potential which'represents' a measure for the mean level of the tone frequency impulse.

If, however, several frequencies are'to be simultaneously transmitted, which in practice is almost always the case, it is no longer possible to separately influence the frequency variation of the individual frequencies, since the direct po- 25 tential is dependent at the same time upon all frequencies.

In accordance with the invention, the circuit is adapted in such manner that with simultaneous transmission of several frequencies. a frequency variation dependent on the transmission level and corresponding to the sensitivity of the ear is obtained. This isaccomplished in the simplest manner by utilizing the condition that with automatic control the internal resistance of a 35 device effecting the control and placed in a frequency responsive network in the amplification train, varies in response to the variation of the amplification.

The invention will be better understood from the following description when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claim.

Referring to the drawing:

Fig. 1 shows a sensitivity curve of the ear as obtained from more recent measurements. As this curve indicates, the maximum sensitivity lies approximately between 1,000 and 4,000 cycles per second. Therefore it will be sufficient if the frequencies below 1,000 cycles are favored by the volume controlling action, and

Fig. 2 is a schematic circuit diagram of a control system embodying the invention.

' If at recording, the control is carried out in such manner that the high signal amplitudes are not influenced or only slightly influenced as regards intensity, while the low signal amplitudes are however increased the impression is produced at the reproduction as if the low frequencies had been favored too extensively. If, for instance, a bass violin and a violin are being played soas to produce the same volume, in the aforedescribed control performance, the bass violin appears to sound essentially louder than the violin. Hence the entire sound picture appears false. This condition can be readily explained with the curves of Fig. 1. If for instance at 1,000 cycles, the level is raised from 30 to 50 db., then when the increase occurs at the same absolute amount the level at 125 cycles will be raised at about 30 db., i. e. a favoring of the low frequencies occurs.

Whether the large amplitudes, the small amplitudes or the mean amplitudes are maintained constant, it is necessary tocause a corresponding influence upon the frequency. As already pointed out, this is accomplished by availing oneself of the fact that in case of automatic control the internal resistance of a device effecting the control, and placed in a network in the amplification train, changes when the amplification varies. Other elements whose function depends upon the frequency are so arranged and connected in circuit with the said device that through the control the frequency will be influenced in accordance with the sensitivity of the ear.

According to Fig. 2, the control device is a nonlinear resistor, or detector or the like.

In the arrangement of the circuit, the automatic control is effected through voltage division between the internal resistance R1 of the voltage source, and at the internal resistance of the detector G1. During the automatic control the resistance of a dry rectifier G1 preferably operating with a biasing potential, varies according to the volume of the alternating voltage to be transmitted. For instance, in recordin the detector is so dimensioned and connected in such manner that at small alternating amplitudes, for obtaining high amplification, the resistance of the detector G1 is high, while at large alternating amplitudes for obtaining a relatively small amplification, the resistance is low.

The frequency dependent control is effected by means of frequency dependent elements cooperating with the controlling detectors. It is advisable to provide the frequency dependent circuit elements with ohmic resistances (in series or in parallel according to the circuit) so as to avoid excessive influences upon certain frequency bands. The elements depending upon the frequency and/or the ohmic resistances are preferably variable, so that varying operating conditions such as variations of the apparent resistance of the detectors and the like, can be taken care of.

In the circuit arrangement according to Fig. 2, a part of the incoming impulses is branched off, and applied to the auxiliary detector Hgl across transformer Tr. The direct current potential produced at resistor R1 and at condenser C1 is applied to the controlling detector G1. There is placed in parallel to the detector a seriesconnection consisting of a frequency-dependent element and of an ohmic'resistor. In the present system which is more particularly adapted for recording, and presupposes the large amplitudes to remain entirely or substantially uncontrolled while the small amplitudes are increased, there is provided an inductance L as a frequencydependent element. If the control is such that the small amplitudes remain practically uncontrolled the said inductance L would have to be replaced by a condenser. A parallel connection consisting of inductance and capacity may also be provided.

The circuit according to Fig. 2 operates in the following manner: In case of large alternating amplitudes, the resistance Rgl of the detector G1. will be comparatively low. As a result thereof a large part of the impulses will be short circuited across detector G1. The shunt network formed of L and R2 remains almost uneffective. However, in case of small amplitudes, the resistance Rgl of the detector G1 becomes comparatively high so that the resistance of the detector itself forms but an unimportant shunt. The high frequencies are transmitted practically without any decrease thereof, while the low frequencies are by-passed across the shunt L-Rz at an adjustable value. In this manner damping curves can be obtained corresponding to the course of the sensitivity curves of the ear.

The aforementioned example shows the circuit arrangement required where at recording, the

' large amplitudes remain practically unconelements should cooperate with the control device in such manner that the desired consideration is given to the sensitivity curve of the ear. As regards the functioning of the arrangement it is immaterial whether a forward or backward control is carried out. In case of forward control it will be advisable to tap the control potentials for the auxiliary rectifier at the grid of the tube so as to avoid disturbances in view of the varying proportion of the voltage division at the inner resistance and at the detector resistance, and which influences the controlperformance when the plate circuit is tapped. The frequencydependent elements should preferably be so dimensioned that the apparent'resistance thereof 1. H. (LOLOI'CTC) at 800 cycles assumes the order of the apparent resistance of the detector at average transmission level. (About 1000 ohms for copper-oxide rectifiers.)

What is claimed is:

Apparatus of the class described comprising an input circuit, an output circuit, 'a' rectifier and a means for providing a source of biasing potential connected in series across said output circuit, and an inductance and a resistor connected in series to each other and in shunt to said rectifier and biasing potential means in said'output circuit, a resistor in series with said output circuit, and a rectifier connected. to said'input circuit and across the resistor in the output circuit for controlling the said shunt circuits in the output circuit in accordance with the volume of audio frequency signal energy, representative of sound, in the input circuit.

HANS BARTELS. 

